Why do LEDs flicker – and how to stop it happening

Why do LEDs flicker – and how to stop it happening

Well… put simply, LED flicker occurs when their light output fluctuates. This fluctuation happens because your dimmable light-emitting diodes are designed to switch on and off at very high speed.

We’ll dig deeper in the article below.

Why do LEDs flicker?

Even though you don’t always see it, all mains-powered light sources flicker – whether incandescent, halogen, fluorescent or LED.

In the UK, mains electricity is an alternating current (AC) supply, delivered at a frequency of 50 hertz. Which means that the electric current that powers your lights – any lights – switches backwards and forwards 50 times each second.

So let’s start by debunking a common misconception: because LED flicker is caused by your electricity supply, it can be avoided completely as long as the power input (current and voltage) remains constant.

This isn’t true. All mains-powered light sources flicker, all the time.

So if all lights flicker, why do some people think flickering LEDs are a problem?

LED flicker used to occur because – unlike incandescents, halogens and fluorescents – LEDs have no persistence. When the power supply to an LED is switched off, the light output stops instantly. So if connected directly to an AC mains-electricity supply, an LED will switch on/off 50 times each second – enough to be visible to the human eye.

That’s why some people think LEDs flicker more than older light sources. But they’re wrong.

Keeping an eye on your LEDs – how a high-quality LED power supply can solve LED flicker

LED lighting requires a direct current (DC) rather than an AC power supply. Which is good news, because the key to eliminating LED flicker is the kind of power-supply you use to drive your lights.

Although the industry generally knows its power supplies (aka ‘drivers’) simply as ‘LED transformers’, they’re actually more than that. An LED driver doesn’t just step down (transform) voltage; it also converts current from mains AC to DC. Choose a high-quality LED power supply, and it will also supply a constant current to your LEDs – so you’ll get light with no visible flicker.

A lower-quality, no-frills LED driver doesn’t provide a constant current though; instead, it simply converts current from AC to DC. This most basic kind of power-supply conversion produces an oscillating current, albeit one that typically doubles the input-voltage frequency. In the UK, that will result in a frequency of 100 potential flickers a second.

100 flickers per sec is obviously much better than 50 flickers per sec. But it still sounds like a problem, doesn’t it? Luckily, in most situations it’s nothing to worry about – because the human eye isn’t perceptive enough to see it. Most of us only register light fluctuating at well below 100 flickers per second – typically 50 or slower. (Computer screens usually flicker in the range of 60 to 70 hertz, which we barely notice.)

So even though there’s certainly a tiny minority of people who can see faster strobing, for most of us it’s not an issue. In many project installations, a basic no-frills LED ‘transformer’ is all you need. There are even some applications where LED flicker can actually be a desirable effect: think of nightclubs for example, or oscillating bicycle lights.

Constant current = constant brightness

But if a simple LED power supply isn’t enough for your project, then an excellent alternative for you would be a constant-current power supply.

These higher-spec LED drivers can practically eliminate flicker, by varying voltage across the circuit in order to generate a constant electric current. This ensures the current delivered to your LEDs doesn’t fluctuate at all, which mitigates the effects of the AC/DC conversion.

There are some situations when even a constant-current LED driver can’t eliminate flicker completely. Interference caused by incompatibility issues with your control circuitry are one common cause; before installing, you should confirm your LED products are suitable for the control circuits and power supply you’re using.

Regular flicker – heed the warning!

If the reason behind your flickering LEDs is interference or incompatibilty, then the flicker effect will be uneven. But if that’s not the case, and the lights are producing a regular, rhythmic flicker (every second, for example) then it’s very likely they are drawing more power than your LED driver is built to supply.

High-quality LED power supplies are designed this way, create the effect as a warning. Regular flickering indicates that your LEDs require a bigger, higher-spec power supply (or several smaller ones).

Loose wiring and other faulty connections can also cause problems.

If you have multiple electronic devices installed in close proximity (on the same switch-plate, for example), the combined heat may be overloading your LED dimmer(s). This is likely to cause intermittent flickering. Australia’s Diginet site has a great article discussing the best way to de-rate overloaded dimmers.

And when LEDs are used for film applications (e.g. TV studios, theatres, etc), you’ll need very high frequency, high resolution outputs beyond the range of a standard driver. If you’re involved in this kind of specialist project, then why not give us a call to discuss the benefits of constant-voltage LED drivers for the film industry?

But for most LED lighting projects and installations…

…the single most frequent cause of visible flicker is the implementation of a dimming function!

Dealing with dimming

Dimming can cause problems even in an otherwise perfect LED lighting installation. That’s because conventional dimmers work by lengthening the ‘off’ part of each on-off flicker cycle, in order to reduce the total amount of light that’s being output.

This technique is called pulse width modulation (PWM). PWM works very effectively – just as long as the switching frequency doesn’t drop to a level that the human eye can perceive.

Some manufacturers are working to solve this problem by developing LED dimmers with a much faster flicker cycle. A cycle of thousands of hertz is the goal, simulating the solution used by the electronic ballasts that have powered fluorescent lighting for many years. There’s a downside, though: the higher the flicker frequency, the nearer to your transformer your LEDs will need to be. Which is not always practical.

Rather than installing these less flexible (and more expensive) dimmers, you can very easily avoid the visible flicker created by pulse width modulation. By simply not dimming your LEDs so low, you’ll prevent their flicker cycle becoming visible to the eye.

Just a few years ago, that often meant not dimming below around 50% of full brightness. But now. the dimmers in our current range all allow you much greater flexibility. You may discover that you can dim all the way without any visible flicker!

Generally you’ll find that zero-to-10V dimmers will be less prone to flicker than, for example, a mains TRIAC control.

Pulse width modulation dimming

Summary – How to stop LEDs flickering

Over the past decade, LEDs have been adopted by the lighting industry as an energy-efficient lighting solution for the future. With all the advantages they offer, that’s not surprising.

But to avoid the effects of LED flicker, you and your electrician will need a basic understanding of the issues behind it. So always keep these points in mind:

1. Always drive LED products using an LED power supply that’s designed for the job. Don’t use any LED strips that are driven by mains AC! 2. Make sure all your LED products are compatible with the control circuits and power supply you’re using. 3. Check for loose wiring and other faulty connections. And make sure your LED dimmers are not overloaded. 4. Consider using a constant-current LED driver. 5. When installing a dimming system, experiment to see if there’s a minimum dimming level that you shouldn’t go below. 6. For dimming systems, consider using a zero-to-10V or digital volt-dimming system rather than a TRIAC alternative.

By Rick Briggs| 2017-11-17T10:39:28+00:00 May 21st, 2015|Categories: News|Comments Off on Why do LEDs flicker – and how to stop it happening